Method for transferring intersystem connections

ABSTRACT

The invention relates to the transfer of intersystem connections, wherein the quality of a transfer to a supplying base station in a first radiocommunication system determined at a given moment by a subscriber station is compared to a threshold value. If the value falls below said threshold, a compressed mode is activated and measurements are carried out by the subscriber station in relation to the transfer qualities to neighbouring base stations in a second radiocommunication network. On the basis of said measurements, the subscriber station selects a base station for the transfer of a connection. If the value falls below the second threshold, information sequences in the organization channels of the selected base stations are detected by the subscriber station.

[0001] The invention relates to a method for transferring intersystemconnections, in particular between two asynchronous radiocommunicationsystems. The invention is particularly suited to use in a mobilecommunication system or wireless subscriber line system.

[0002] In radiocommunication systems, for example in the secondgeneration European mobile communication system GSM (Global System forMobile Communications), information such as voice, image information orother data for example is transmitted via a radio interface usingelectromagnetic waves. The radio interface relates to a connectionbetween a base station and a plurality of subscriber stations, whereinthe subscriber stations may be mobile stations or fixed radio stationsfor example. The electromagnetic waves are emitted in this case usingcarrier frequencies lying in a frequency band provided for therespective system. For future radiocommunication systems, for examplethe UMTS (Universal Mobile Telecommunications System) or other thirdgeneration systems, frequencies are provided in the frequency band ofapproximately 2000 MHz. Two modes are provided for the third generationmobile communication system UMTS, with one mode delineating an FDD(frequency division duplex) mode and the other mode delineating a TDD(time division duplex) mode. Said modes are used in different frequencybands, with both modes supporting a so-called CDMA (Code DivisionMultiple Access) subscriber separation method.

[0003] For the description of the prior art with respect to theobservation of GSM radio cells based on the FDD mode of the thirdgeneration digital mobile communication system UMTS at the time ofapplication, the following documents form the basis for international 3GPP standardization:

[0004] D1: TS 25.212 “Multiplexing and channel coding (FDD)”, V3.1.1,1999-12, especially Chapter 4.4 “Compressed mode”,

[0005] D2: TS 25.215 “Physical layer—Measurements (FDD)”, V3.1.1,1999-12, especially Chapter 6 “Measurements for UTRA FDD”, and

[0006] D3: RAN 25.231 “Physical layer—Measurements”, VO.3.0, 1999-06,especially Chapter 5.1.3 ff. “Measurements for the handover preparationfrom UTRA FDD at the UE”.

[0007] As the general prior art, descriptions of the second generationmobile communication system GSM are based on the book by J. Biala“Mobilfunk und Intelligente Netze”, Vieweg Verlag, 1995.

[0008] Owing to a co-existence and a desired harmonization betweensecond and third generation radiocommunication systems, subscriberstations that have established a connection in one radiocommunicationsystem should be provided with the capability to transfer the connectionto a further radiocommunication system which in some cases may support adifferent transmission mode. An intersystem connection transfer of thistype, also referred to as intersystem handover, requires that prior tothe transfer the subscriber station must already be synchronized withthe radiocommunication system that is to take over the connection. Forthis reason, signals of a so-called synchronization channel (SCH) aretransmitted periodically by the base stations of the radiocommunicationsystem in the radio coverage area, by means of which signals asubscriber station can synchronize with the time structure of the radiointerface of the radiocommunication system and can subsequently carryout measurements, for example regarding the receiving level, which aretaken into account for the transfer decision.

[0009] The FDD mode of the UMTS mobile communication system is based ona so-called W-CDMA subscriber separation method which is characterizedby continuous transmitting and receiving on designated broadbandtransmission channels. In contrast to the known time slot structure ofthe GSM mobile communication system and of the TDD mode of the UMTSmobile communication system, no dedicated transmission pauses formeasuring adjacent radio cells or parallel mobile communication systemsoperating in a different frequency band are available to a subscriberstation in the FDD mode when changing over between transmitting andreceiving.

[0010] One solution to this problem is the realization of a secondreceiving device in the subscriber station, but this disadvantageouslyresults in increased costs, an additional space requirement as well as ahigher power consumption of the subscriber station.

[0011] For this reason a concept was realized according to which asubscriber station having only one receiving device is also capable ofdetecting signals in other frequency bands, and of using them forexample for an intrasystem or intersystem connection transfer. Thisconcept is termed “compressed mode” and is explained inter alia in thereferenced documents D1 to D3. With this mode, within a time frame of 10ms, the information contained therein is compressed, inter alia byvarious methods such as puncturing and changing the spreading factor, insuch a way that a transmission gap of a specific length is produced.Within said transmission gap, the subscriber station can tune thereceiving device to another frequency band and receive and evaluatesignals transmitted therein. The “compressed mode” can be performed bothin the uplink direction and in the downlink direction.

[0012] However, this concept also has disadvantages because, forexample, a higher transmitting power becomes necessary as a result ofthe reduction of the spreading factor in order to ensure a constanttransmission quality. This increased transmitting power leads toincreased interference disruption between connections concurrentlyestablished in the same frequency band.

[0013] Moreover, the concept disadvantageously breaks the closed loopfor transmit power control. This runs counter to the principle of aDS-CDMA system (Direct Sequence CDMA) , which requires a very fast andprecise transmit power control for the uplink direction to ensureoptimal capacity of the system by minimizing the respective transmittingpower of the subscriber stations.

[0014] The number and periodicity of the time frames with transmissiongaps are individually adjusted on the network side depending on therespective conditions and the current need for observing other frequencybands or systems.

[0015] As the future UMTS mobile communication system starts to becomewidespread, so-called multimode subscriber stations will support atleast both the GSM standard and the FDD mode of the UMTS standard. Thisis important primarily for operators who implement, for example, bothcomprehensive coverage of an entire country with a GSM mobilecommunication system and an initially locally restricted coverage withthe UMTS mobile communication system.

[0016] In comparison to the UMTS mobile communication system, the GSMmobile communication system has a significantly smaller frequencychannel spacing—200 kHz in comparison to 5 MHz with FDD mode—as well asa greater frequency reuse factor—typically 7 as opposed to 1. Thisrequires the observation of a greater number of adjacent radio cells,which must be observed in the case of an intersystem connection transferfrom an FDD mode to a GSM system.

[0017] According to the GSM standard, for example the receiving level(RSSI—Received Signal Strength Indicator) of up to 32 adjacent cellsmust be observed by the subscriber station within a period of 30seconds, and the six adjacent cells that offer the best transmissionconditions must be signaled every 480 milliseconds to the currentlysupplying base station. In addition to this observation of therespective receiving levels, information of the respective controlchannel (BCCH—Broadcast Control Channel) must also be decoded andevaluated.

[0018] In the GSM mobile communication system, this problem is solved byaveraging the measured RSSI within a respective time frame (4.6 ms) andby using a so-called idle frame, that is to say a time frame in which notransmission takes place, for detecting information of a selected radiocell.

[0019] In contrast, a subscriber station with a connection establishedin the FDD mode of the UMTS standard has no recourse to suchconcentrated measurements since a regular use of the compressed modewould lead to a significant reduction in the transmission quality. Forthis reason, it is anticipated that no generation of transmission gapswith a periodicity of 120 ms will be provided in the FDD mode.

[0020] The transmission gaps can however be used to observe a pluralityof frequency bands in each case. In comparison with high-periodicityobservation, this is more efficient since the required times forcontrolling the receiving device cause corresponding losses.Nevertheless, one entire transmission gap should be used exclusively fordetecting the information of the control channel of an adjacent GSMradio cell.

[0021] Owing to the negative impact on the transmission quality setforth above, the compressed mode is not used permanently but rather, forexample, the beginning and the extent of the measurements for acalculated need, to maintain an established connection for example, aredetermined and signaled to the subscriber station. For this decisionregarding an activation or deactivation of the compressed mode, the useof a threshold value with which the respective current transmissionquality of the connection is compared is proposed.

[0022] One disadvantage of this solution for controlling the connectiontransfer from the FDD mode to the GSM standard lies in its need for alarge number of measurements with a concomitant decoding of information,resulting inter alia from the large frequency reuse factor and the largenumber of frequency channels.

[0023] With the aid of a diagram, FIG. 2 illustrates the effect ofshifting the threshold value Th for the transmission quality. Thetransmission quality Q is plotted here against a signal-to-noise ratio(Eb/No), where Eb represents an averaged energy of an information bit ofa transmission channel. The compressed mode is activated when the valuefalls below the respective threshold.

[0024] As can be seen from the diagram, with compressed mode activatedit is only possible to achieve a poorer quality for the same Eb/Noratio, with a respective performance loss as a result.

[0025] If the threshold value is set low, as in example b of FIG. 2,then a large number of transmission gaps per time unit is required,which leads to a clear performance constraint as described. The largenumber of transmission gaps is necessary to perform all RSSImeasurements and decoding in as short as possible a time before apossible loss of the connection. In the case where the subscriberstation is near the radio cell boundaries and the transmission qualityis generally already very low, this can lead to a premature loss of theconnection.

[0026] If, on the other hand, the threshold value is set high, as inexample a of FIG. 2, the additional degradation of quality results fromcompressed mode being activated at an earlier time, but in this casethere is sufficient time available for recording and evaluating all thenecessary measurements and information. In comparison with example b,this leads to more reliable information about adjacent radio cellspotentially suitable for the connection transfer.

[0027] Current proposals proceed from a periodic insertion oftransmission gaps into the continuous data transmission, with thetransmission gaps being used in each case for RSSI determination anddecoding of the information sequences of the control channels. However,if said transmission gaps are inserted only with a low periodicity, thenthe reliability of the information about potentially suitable adjacentradio cells is in turn reduced, and the probability increases of adecoding of radio cells having too low a transmission quality within thebest six radio cells determined by the subscriber station.

[0028] The object of the invention is to disclose a method whichrealizes a more efficient and more reliable method for activating thecompressed mode on the basis of the known method. This object isachieved by the method having the features of patent claim 1.Advantageous further developments of the invention emerge from thedependent patent claims.

[0029] The method according to the invention will advantageously use inparticular in the case where the first radiocommunication systemsupports an FDD transmission method. The knowledge of the time structureof the synchronization channel of the second radiocommunication system,which supports a GSM transmission method for example, can be used herefor targeted insertion of transmission pauses, so-called transmissiongaps, into the continuous data transmission in order to receive andevaluate the synchronization channel of the parallel radiocommunicationsystem. In the case of a CDMA subscriber separation method, thetransmission pauses are generated for example as described above by areduction of the spreading factor or by a puncturing of the data to betransmitted. This requires a higher transmitting power for the rest ofthe data, which in turn worsens the interference situation in the radiocell. A restriction of the number of transmission pauses by thetwo-stage detection of base stations of the further radiocommunicationsystem thus advantageously permits an increase in the transmissioncapacity as well as an improvement of the transmission quality.

[0030] It is particularly advantageous if the method according to theinvention described is employed in a radiocommunication system realizedas a mobile communication system or wireless subscriber line system.

[0031] Exemplary embodiments of the invention are described in greaterdetail with reference to the accompanying drawings, in which:

[0032]FIG. 1 shows a block circuit diagram of two adjacentradiocommunication systems;

[0033]FIG. 2 shows an exemplary diagram for the method according to theprior art;

[0034]FIG. 3 shows an exemplary diagram corresponding to the methodaccording to the invention; and

[0035]FIG. 4 shows a temporal flow chart of the method according to theinvention.

[0036]FIG. 1 shows sections in each case from two mobile communicationsystems RS1, RS2 as examples of radiocommunication systems. A mobilecommunication system is composed in each case of a plurality of mobileswitching centers MSC or UMSC (UMTS Mobile Switching Center) whichbelong to a switching network (SSS—Switching Subsystem) and areinternetworked, or establish access to a fixed network, and of one ormore base station systems BSS (Base Station Subsystem) connected to saidmobile switching centers MSC, UMSC in each case. A base station systemBSS has in turn at least one device BSC (Base Station Controller) or RNC(Radio Network Controller) for assigning radio resources, as well as atleast one base station BTS (Base Transceiver Station) or NB (node B)connected thereto in each case. A base station BTS, NB can establishconnections via a radio interface to subscriber stations UE (UserEquipment), such as mobile stations or other mobile and stationaryterminals for example. At least one radio cell Z is formed by each basestation BTS, NB. The size of the radio cell is usually determined by therange of a general signaling channel BCH (Beacon Channel) or BCCH(Broadcast Control Channel), which is transmitted by the base stationsBTS, NB with a higher transmitting power than the traffic channels ineach case. In the case of sectorization or with hierarchical cellstructures, it is also possible to supply a plurality of radio cells perbase station BTS, NB. The functionality of this structure can betransferred to other radiocommunication systems in which the inventioncan be employed, in particular to subscriber access networks withwireless subscriber lines.

[0037] The example of FIG. 1 shows a subscriber station UE which isdesigned as a mobile station and which moves at a speed V from the radiocell Z of the first mobile communication system RS1, which supports anFDD mode of the UMTS standard, to a radio cell Z of a second mobilecommunication system RS2, which supports a GSM standard. The subscriberstation UE has established a connection to the base station NB, shown byway of example, of the first mobile communication system RS1. During theconnection the subscriber station UE periodically evaluates thetransmission conditions of the radio interface to its surrounding basestations, such as the aforementioned base station NB of the secondmobile communication system RS2 for example, in order, in the event ofthe transmission quality to the base station NB of the first mobilecommunication system RS1 becoming degraded for example, to initiate aconnection transfer to the base station BTS of the second mobilecommunication system RS2 with better transmission characteristics. Thesame method is used for example also in hierarchical network structureswhen a connection between two different hierarchical levels, for examplefrom a microcell into a macrocell, that are operating in a differentfrequency band in each case, is transferred.

[0038] In future radiocommunication systems, such as the UMTS mobilecommunication system for example, it should also be possible toimplement said connection transfer between radiocommunication systemsthat support different transmission methods. In this case a voiceconnection can be transferred from, for example, an FDD system to a GSMsystem or from a TDD system of a lower hierarchical level of thehierarchical network structure to an FDD or GSM system of a higherhierarchical level. Further scenarios of connection transfer between thesame or different systems and transmission methods are conceivable.

[0039] To illustrate the method according to the invention, FIG. 3 showsby way of example a diagram that corresponds to the diagram in FIG. 2initially described. In contrast to the known method with a comparisonof the determined transmission quality (Q) of the connection to thecurrently supplying base station with a threshold value, according tothe invention the currently determined transmission quality is comparedwith two threshold values Th1 and Th2 (Threshold).

[0040] If the transmission quality falls below a first threshold valueTh1, then the compressed mode is activated at least for transmission inthe downlink direction and is signaled to the subscriber station. Incontrast to the known method of FIG. 2, the subscriber station UEdetermines only the RSSI of the surrounding base stations, for exampleaccording to a signaled adjacent cell list, in the quality intervalbetween the first Th1 and the second threshold value Th2.

[0041] The fact that a plurality of adjacent cells can be observedduring a transmission gap enables the period between two successivetransmission gaps to be increased up to an upper limit which stillpermits sufficient reliability of the evaluation and sequence of theobserved adjacent cells. Accordingly, the interval PL (Performance Loss)between the two threshold values Th1 and Th2 serves to determine a listwith a sequence, for example determined and if appropriate evaluatedaccording to the respective transmission quality, of adjacent radiocells or base stations potentially suitable for a connection transfer.For example the subscriber station UE creates here a list with the sixmost suitable base stations.

[0042] If the transmission quality subsequently falls below the secondthreshold value Th2, then the subscriber station UE decodes informationsequences transmitted by the base stations of the selected adjacentradio cells in a respective control channel. In this case the RSSI is nolonger determined, which confers the advantage that no increase in theperiodicity for generating the transmission gaps is required. Theperiodicity already selected for RSSI determination can also be usedfurther for the decoding, so that no further degradation of thetransmission quality arises.

[0043] The advantageous achievement of the method according to theinvention is that it requires no compromise between RSSI measurementsand decoding of information sequences. The concept optimizes the methodwith respect to a smallest possible periodicity of the transmission gapswith at the same time a reliable ranking of the adjacent cellcandidates. The less efficient decoding procedure is only activated ifthe transmission quality falls below the second threshold value Th2, butsaid procedure then utilizes the transmission gaps exclusively. Byvirtue of the fact that all actions between the first threshold valueTh1 and second threshold value Th2 serve to determine the RSSI, thereliability of the decoding of relevant adjacent cell candidates isadvantageously increased.

[0044]FIG. 3 shows the method according to the invention as a temporalflow chart in accordance with the preceding description of FIG. 2.

1. A method for controlling an intersystem connection transfer from afirst radiocommunication system (RS1) having a first transmission method(FDD) to a second radiocommunication system (RS2) having a secondtransmission method (TDD, GSM), characterized in that a transmissionquality (Q) determined by a subscriber station (UE) with respect totransmission characteristics to a supplying base station (NB) of thefirst radiocommunication system (RS1) is compared with a first thresholdvalue (Th1), a compressed mode method is activated if the value fallsbelow the first threshold (Th1) , wherein, within at least onetransmission gap generated in a time frame of the first transmissionmethod (FDD), the subscriber station (UE) determines transmissioncharacteristics from/to at least one base station (BTS) of the secondradiocommunication system (RS2), the subscriber station (UE) detectsinformation sequences transmitted by the base station (BTS) of thesecond radiocommunication system (RS2) in a control channel (BCCH) ifthe value falls below a second threshold (Th2), and on the basis of themeasurements, the subscriber station (UE) selects at least one basestation (BTS, NB) suitable for the transfer of a connection.
 2. Themethod as claimed in claim 1, characterized in that a respectivereceiving level (RSSI) is determined as transmission quality (Q) by thesubscriber station (UE).
 3. The method as claimed in claim 1 or 2,characterized in that a periodicity for the generation of thetransmission gaps is selected depending on a current transmissionquality (Q) from/to the supplying base station (NB) of the firstradiocommunication system (RS1).
 4. The method as claimed in one of thepreceding claims, characterized in that the start and/or the periodicityfor the generation of the transmission gap is determined by the basestation (NB) of the first radiocommunication system (RS1) and signaledto the subscriber station (UE).
 5. The method as claimed in one of thepreceding claims, characterized in that the compressed mode method isactivated for a signal transmission in the uplink and/or in the downlinkdirection.
 6. The method as claimed in one of the preceding claims,characterized in that the measurements of the subscriber station (UE)are signaled together with further characteristic values abouttransmission characteristics of the radio interface between the basestation (NB) of the first radiocommunication system (RS1) and thesubscriber station (UE) and/or between a base station (BTS) of thesecond radiocommunication system (RS2) and the subscriber station (UE)????? signaled to whom ?????.
 7. The method as claimed in one of thepreceding claims, characterized in that the first radiocommunicationsystem (RS1) supports an FDD transmission method (FDD), and the secondradiocommunication system (RS2) supports a TDD transmission method (TDD)or a GSM transmission method (GSM).
 8. A base station system (BSS) of aradiocommunication system (RS1, RS2) for carrying out the method asclaimed in one of the preceding claims, characterized in that theradiocommunication system (RS1, RS2) is realized as a mobilecommunication system or as a wireless subscriber line system.